"Apple built upon its strong March iPad launch and ended the quarter with its best-ever shipment total for the iPad, outrunning even the impressive shipment record it set in the fourth quarter of last year," said Tom Mainelli, an IDC analyst.

"The vast majority of consumers continue to favor the iPad over competitors."

Demand is increasingly strong in sectors such as education, said Mainelli.

"While iPad shipment totals are beginning to slow a bit in mature markets where the device saw early traction, growth in other regions is clearly more than making up the difference," he added.

Amazon rebounded from a sluggish first quarter to sell 1.2 million of its Kindle Fire, which was launched late last year and is sold only in the United States.

Asus was fourth with 855,000 tablets sold, a jump of 115% from a year earlier.

The data do not include the Google-Asus co-branded Nexus 7, which was launched in July.

IDC said it expects competition in the tablet market to continue to heat up in the second half of 2012 with new product launches from Amazon, probably Apple, and an influx of Microsoft Windows 8 and Windows RT-based tablets.

"If anything, there's a real risk that people will have too many options from which to choose this holiday season," said Bob O'Donnell of IDC.

"Consumers baffled by the differences between Amazon and Google versions of Android, or Windows 8 and Windows RT, may well default to market leader Apple. Or they may simply choose to remain on the sideline for another cycle."

Many analysts believe Apple will launch a smaller version of its iPad later this year, and that Amazon will release an upgraded Kindle Fire. Microsoft Corp. (IW 500/15) is set to release its Surface tablet in late October.